The speed at which
light travels through a vacuum, about 186,000 miles per second, is enshrined
in physics lore as a universal speed limit. Nothing can travel faster than
that speed, according to freshman textbooks and conversation at sophisticated
wine bars; Einstein's theory of relativity would crumble, theoretical physics
would fall into disarray, if anything could.
Two new experiments have
demonstrated how wrong that comfortable wisdom is (although physicists
say Einstein's theory survives).
In the most striking of the
new experiments a pulse of light that enters a transparent chamber filled
with specially prepared cesium gas is pushed to speeds of 300 times the
normal speed of light. That is so fast that, under these peculiar circumstances,
the main part of the pulse exits the far side of the chamber even before
it enters at the near side.
It is as if someone looking
through a window from home were to see a man slip and fall on a patch of
ice while crossing the street well before witnesses on the sidewalk saw
the mishap occur -- a preview of the future. But Einstein's theory, and
at least a shred of common sense, seem to survive because the effect could
never be used to signal back in time to change the past -- avert the accident,
in the example.
A paper on the experiment,
by Lijun Wang of the NEC Research Institute in Princeton, N.J., has been
submitted to Nature and is currently undergoing peer review. It
is only the most spectacular example of work by a wide range of researchers
recently who have produced superluminal speeds of propagation in various
materials, in hopes of finding a chink in Einstein's armor and using the
effect in practical applications like speeding up electrical circuits.
"It looks like a beautiful
experiment," said Raymond Chiao, a professor of physics at the Univ. of
California/Berkeley (Berkeley, CA), who, like a number of physicists in
the close-knit community of optics research, is knowledgeable about Dr.
Wang's work.
Dr. Chiao, whose own research
laid some of the groundwork for the experiment, added that "there's been
a lot of controversy" over whether the finding means that actual information
-- like the news of an impending accident -- could be sent faster than
c, the velocity of light. But he said that he and most other physicists
agreed that it could not.
Though declining to provide
details of his paper because it is under review, Dr. Wang said, "Our light
pulses can indeed be made to travel faster than c. This is a special property
of light itself, which is different from a familiar object like a brick,"
since light is a wave with no mass. A brick could not travel so fast without
creating truly big problems for physics, not to mention humanity as a whole.
A paper on the second new
experiment, by Daniela Mugnai, Anedio Ranfagni and Rocco Ruggeri of the
Italian National Research Council, described what appeared to be slightly
faster-than-c propagation of microwaves through ordinary air, and was published
in the May 22 issue of Physical Review Letters.
The kind of chamber in Dr.
Wang's experiment is normally used to amplify waves of laser light, not
speed them up, said Aephraim M. Steinberg, a physicist at the Univ. of
Toronto (Toronto CN). In the usual arrangement, one beam of light is shone
on the chamber, exciting the cesium atoms, and then a second beam passing
through the chamber soaks up some of that energy and gets amplified when
it passes through them.
But the amplification occurs
only if the second beam is tuned to a certain precise wavelength, Dr. Steinberg
said. By cleverly choosing a slightly different wavelength, Dr. Wang induced
the cesium to speed up a light pulse without distorting it in any way.
"If you look at the total pulse that comes out, it doesn't actually get
amplified," Dr. Steinberg said.
There is a further twist
in the experiment, since only a particularly strange type of wave can propagate
through the cesium. Waves Light signals, consisting of packets of waves,
actually have two important speeds: the speed of the individual peaks and
troughs of the light waves themselves, and the speed of the pulse or packet
into which they are bunched. A pulse may contain billions or trillions
of tiny peaks and troughs. In air the two speeds are the same, but in the
excited cesium they are not only different, but the pulses and the waves
of which they are composed can travel in opposite directions, like a pocket
of congestion on a highway, which can propagate back from a toll booth
as rush hour begins, even as all the cars are still moving forward.
These so-called backward
modes are not new in themselves, having been routinely measured in other
media like plasmas, or ionized gases. But in the cesium experiment, the
outcome is particularly strange because backward light waves can, in effect,
borrow energy from the excited cesium atoms before giving it back a short
time later. The overall result is an outgoing wave exactly the same in
shape and intensity as the incoming wave; the outgoing wave just leaves
early, before the peak of the incoming wave even arrives.
As most physicists interpret
the experiment, it is a low-intensity precursor (sometimes called a tail,
even when it comes first) of the incoming wave that clues the cesium chamber
to the imminent arrival of a pulse. In a process whose details are poorly
understood, but whose effect in Dr. Wang's experiment is striking, the
cesium chamber reconstructs the entire pulse solely from information contained
in the shape and size of the tail, and spits the pulse out early.
If the side of the chamber
facing the incoming wave is called the near side, and the other the far
side, the sequence of events is something like the following. The incoming
wave, its tail extending ahead of it, approaches the chamber. Before the
incoming wave's peak gets to the near side of the chamber, a complete pulse
is emitted from the far side, along with a backward wave inside the chamber
that moves from the far to the near side.
The backward wave, traveling
at 300 times c, arrives at the near side of the chamber just in time to
meet the incoming wave. The peaks of one wave overlap the troughs of the
other, so they cancel each other out and nothing remains. What has really
happened is that the incoming wave has "paid back" the cesium atoms that
lent energy on the other side of the chamber.
Someone who looked only at
the beginning and end of the experiment would see only a pulse of light
that somehow jumped forward in time by moving faster than c.
"The effect is really quite
dramatic," Dr. Steinberg said. "For a first demonstration, I think this
is beautiful."
In Dr. Wang's experiment,
the outgoing pulse had already traveled about 60 feet from the chamber
before the incoming pulse had reached the chamber's near side. That distance
corresponds to 60 billionths of a second of light travel time. But it really
wouldn't allow anyone to send information faster than c, said Peter W.
Milonni, a physicist at Los Alamos National Laboratory. While the peak
of the pulse does get pushed forward by that amount, an early "nose" or
faint precursor of the pulse has probably given a hint to the cesium of
the pulse to come.
"The information is already
there in the leading edge of the pulse," Dr. Milonni said. "You can get
the impression of sending information superluminally even though you're
not sending information."
The cesium chamber has reconstructed
the entire pulse shape, using only the shape of the precursor. So for most
physicists, no fundamental principles have been smashed in the new work.
Not all physicists agree
that the question has been settled, though. "This problem is still open,"
said Dr. Ranfagni of the Italian group, which used an ingenious set of
reflecting optics to create microwave pulses that seemed to travel as much
as 25 percent faster than c over short distances.
At least one physicist, Dr.
Günter Nimtz of the Univ. of Cologne, holds the opinion that a number
of experiments, including those of the Italian group, have in fact sent
information superluminally. But not even Dr. Nimtz believes that this trick
would allow one to reach back in time. He says, in essence, that the time
it takes to read any incoming information would fritter away any temporal
advantage, making it impossible to signal back and change events in the
past.
However those debates end,
Dr. Steinberg said that techniques closely related to Dr. Wang's might
someday be used to speed up signals that normally get slowed down by passing
through all sorts of ordinary materials in circuits. A miniaturized version
of Dr. Wang's setup "is exactly the kind of system you'd want for that
application," Dr. Steinberg said.
Sadly for those who would
like to see a computer chip without a speed limit, the trick would help
the signals travel closer to the speed of light, but not beyond it, he
said.
